Nickel base superalloys are widely used in applications where high stresses must be endured at elevated temperatures. One such application is the field of gas turbine engines where nickel base superalloys are widely used especially for blades and vanes. Demands for improved efficiency and performance have resulted in the operation of turbine engines at increasingly elevated temperatures placing extreme demands on the superalloy articles used therein.
One approach which has been successfully employed to improve the temperature capabilities of nickel base alloys is to fabricate them in the form of single crystals. Conventionally prepared metallic materials are comprised of a plurality of grains which are separated by grain boundaries. The grain boundaries are weak at elevated temperatures, much weaker than the material within the grains. Through specific casting techniques nickel base superalloys can be produced in single crystal form, that is, in the form of articles which have no internal grain boundaries. U.S. Pat. No. 3,260,505 describes nickel base single crystal superalloy articles. U.S. Pat. No. 4,116,723 describes a heat treatment method applicable to a class of nickel single crystal articles. U.S. Pat. No. 4,209,348 describes a more specific class of single crystal articles and describes a method for heat treating such articles to improve their elevated temperature mechanical properties. The nominal composition disclosed in this patent is 10% chromium, 5% aluminum, 1.5% titanium, 4% tungsten, 12% tantalum, 5% cobalt, balance essentially nickel. Single crystals of this composition are used commercially in gas turbine engine applications. This composition is generally acknowledged to have the best overall combination of properties attainable in a cast nickel base superalloy article. Another single crystal superalloy composition is that which is described in U.S. Pat. No. 4,402,772 having a nominal composition of 6% tantalum, 9% chromium, 5% cobalt, 1% titanium, 1% molybdenum, 7% tungsten, 5.5% aluminum, 0.15% hafnium, balance essentially nickel. This material also has desirable properties which are essentially equivalent to those of the properties in the U.S. Pat. No. 4,209,348 material. Yet another composition is that described in U.S. Pat. No. 4,222,794 having a nominal composition of 5.2% chromium, 5.4% aluminum, 1.1% titanium, 2% molybdenum, 4.9% tungsten, 6.4% tantalum, 3% rhenium, 0.4% vanadium, balance essentially nickel.